Radiological Aspects of Deep-Burn Fusion-Fission Hybrid Waste in a Repository
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Radiological Aspects of Deep-Burn Fusion-Fission Hybrid Waste in a Repository Henry F. Shaw, James A. Blink, Joseph C. Farmer, Kevin J. Kramer, Jeffery F. Latkowski, and Pihong Zhao Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, U.S.A. ABSTRACT The quantity, radioactivity, and isotopic characteristics of the spent fission fuel from a hybrid fusion-fission system capable of extremely high burnups are described. The waste generally has higher activity per unit mass of heavy metal, but much lower activity per unit energy generated. The very long-term radioactivity is dominated by fission products. Simple scaling calculations suggest that the dose from a repository containing such waste would be dominated by 129I, 135Cs, and 242Pu. Use of such a system for generating energy would greatly reduce the need for repository capacity INTRODUCTION Lawrence Livermore National Laboratory is developing a hybrid fusion-fission nuclear energy system, called LIFE (Laser Inertial-confinement Fusion-fission Engine) to generate electrical power and burn nuclear waste [1, 2, 3]. The system uses the neutrons produced by laser-driven inertial confinement fusion to produce additional fusion fuel and to drive nuclear reactions in a subcritical fission blanket. The fusion neutron source obviates the need for a selfsustaining chain reaction in the fission blanket. Both fissile fuels (e.g., 235U, 239Pu), and fertile fuels (e.g., depleted uranium, natural uranium, 232Th, or spent LWR fuel) could be used as fission fuel, thus eliminating the need for isotopic enrichment. The fusion neutrons allow extremely high levels of burnup to be reached, extracting a large fraction of the available energy in the fission fuel without the need for reprocessing. In this paper, we discuss the radionuclide inventory of a depleted uranium (DU) fuel burned to greater than 95 % FIMA (Fissions per Initial heavy Metal Atom), and the implications of the resulting waste relative to dose standards for releases from a geological repository for high-level waste. The LIFE system producing the waste discussed here uses low-yield (37.5 MJ), deuterium-tritium fusion targets ignited at a rate of 13.3 Hz to produce a 500 MW fusion source. Of this energy, ~400 MW is carried off by 14 MeV neutrons (1.8 × 1020 n/s), and the remaining energy is carried off in ions and x-rays. The fusion neutrons are multiplied and moderated by a sequence of concentric shells of materials before encountering the fission fuel. The fission blanket contains, in this case, 40 metric tons (MT) of DU fuel. For this analysis, the fuel was assumed to be TRISO-like UOC fuel particles embedded in 2-cm-diameter graphite pebbles. (Present TRISO fuel designs may not reach the high burnups of the fertile fuel considered here, and other fuel options, including molten salt fuel, are being investigated. Here, we assume the existence of a fuel that can reach >95% FIMA.) The fission fuel pebbles are cooled by a molten LiF-BeF2 (flibe) coolant, which also produces tritium for th
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